Electrically-actuated linear pump system and method

ABSTRACT

An electrically-actuated linear pump includes a linear actuator housing coupled to a screw housing. A rod is configured to linearly translate within screw housing and the linear actuator housing, the rod includes a threaded portion disposed axially between a first plunger portion and a second plunger portion. A linear actuator is disposed within the linear actuator housing and includes a drive mechanism in meshed engagement with the threaded portion of the rod, the meshed engagement operable to linearly translate the rod, the linear actuator being electrically-actuated. The pump includes a first fluid end and a second fluid end disposed opposite the first fluid end, each fluid end configured to receive and discharge a fluid. Translation of the rod toward the first fluid end discharges the fluid from the first fluid end and simultaneously draws the fluid into the second fluid end.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 62/863,142, filed on Jun. 18, 2019 and entitled “LINEAR ELECTRIC PUMP,” the disclosure of which is incorporated herein by reference in its entirety.

FIELD

This disclosure relates in general to linear pumps, and more particularly to electrically-actuated linear pumps for use in oil and natural gas extraction.

BACKGROUND

Large pumps are commonly used for mining and oilfield applications, such as, for example, hydraulic fracturing. During hydraulic fracturing, fracturing fluid (i.e., cement, mud, frac sand and other material) is pumped at high pressures into a wellbore to cause the producing formation to fracture. One commonly used pump in hydraulic fracturing is a high pressure reciprocating pump, like the SPM® Destiny™ TWS 2500 frac pump or the SPM® QEM 3000 Continuous Duty Frac Pump, manufactured by S.P.M. Flow Control, Inc. of Fort Worth, Tex. In operation, the fracturing fluid is caused to flow into and out of a pump fluid chamber as a consequence of the reciprocation of piston-like plungers, each respectively moving away from and toward the fluid chamber. As a plunger moves away from the fluid chamber, the pressure inside the chamber decreases, creating a differential pressure across an inlet valve, drawing the fracturing fluid through the inlet valve into the chamber. When the plunger changes direction and begins to move towards the fluid chamber, the pressure inside the chamber substantially increases, which closes the inlet valve, and the pressure continues to increase until the differential pressure across an outlet valve causes the outlet valve to open, enabling the highly pressurized fracturing fluid to discharge through the outlet valve into the wellbore.

A typical frac unit is powered with a diesel engine driving a frac pump through a multispeed transmission. The rotational energy transferred to the reciprocating frac pump is channeled to horizontal plunger bores for pumping via crankshaft and conrods. The operating conditions are often extreme involving high fluid flow and high operating pressures (oftentimes up to 15,000 psi). Pressure fluctuations as seen in diesel powered units or other internal combustion based units often cause undesirable cyclic stresses on components, shortening their lives.

Linear pumps may be employed in oilfield operations. For example, PCT Patent App. Publication No. WO 2019/007774 of RSM Imagineering AS, and entitled “A Dual-Acting Pressure Boosting Liquid Partition Device, System, Fleet and Use,” filed on Jun. 27, 2018 discloses a hydraulic linear pump, where hydraulic fluid displaces a rod to actuate a dual-acting pump.

SUMMARY

This disclosure presents an electrically-actuated linear pump powered by an electric motor, a system using two or more such pumps, and a method implementing such a system to provide a uniform pumping flow rate.

The introduction of natural gas as “free fuel” for the frac job has led to investigation of the best method to turn natural gas into frac pumping power. One option for a prime mover is a large gas turbine generator that creates electrical power to run the frac job on electricity. Since electric drive is not limited to the maximum diesel engine power feasible for a mobile frac unit, a larger pump is possible. Larger pumps would lead to less units required on location. Less units on location has a lower total cost of ownership.

Reciprocating pumps have many moving parts as do the power systems that drive them. Replacing reciprocating pumps and their associated drive systems with electrically-actuated linear pumps actuated electrically through a planetary roller screw drive provides many advantages. The linear pumping action is created by the movement of the screw through the electrically driven planetary drive.

An electrically-actuated linear pump includes a linear actuator housing coupled to a screw housing. A rod is configured to linearly translate within the screw housing. The rod includes a threaded portion disposed axially between a first plunger portion and a second plunger portion. A linear actuator is disposed within the linear actuator housing and includes a drive mechanism in meshed engagement with the threaded portion of the rod, the meshed engagement operable to linearly translate the rod, the linear actuator being electrically-actuated. The pump includes a first fluid end and a second fluid end disposed opposite the first fluid end, each fluid end configured to receive and discharge a fluid. Translation of the rod toward the first fluid end discharges the fluid from the first fluid end and simultaneously draws the fluid into the second fluid end.

DESCRIPTION OF THE FIGURES

The accompanying drawings facilitate an understanding of the various embodiments.

FIG. 1 is a top view of an electrically-actuated linear pump system employing three electrically-actuated linear pumps.

FIGS. 2A, 2B, and 2C are schematic diagrams illustrating plunger displacement of an embodiment of a pump of the electrically-actuated linear pump system of FIG. 1.

FIG. 3. 3 is a schematic diagram of components of a linear actuator and planetary gear train of an electrically-actuated linear pump according to the teachings of the present disclosure.

FIG. 4 is a schematic diagram of a power system that may be employed to provide electric power to an electrically-actuated linear pump according to the teachings of the present disclosure.

FIG. 5 is a perspective view of an embodiment of an electrically-actuated linear pump according to the teachings of the present disclosure.

Like numerals refer to like elements.

DETAILED DESCRIPTION

This disclosure presents an electrically-actuated linear pump, a system having an array thereof, and a control method using such pump(s) to provide a constant or steady output flow rate. In traditional reciprocating pumps, many moving parts are involved, including the moving parts in the power systems that drive the reciprocating pumps. The power systems are often driven by an internal combustion engine. As such, efficiency and maintenance costs are high. The present disclosure instead utilizes electrically-actuated linear pumps to simplify the overall system complexity while maintaining power and reliability. This improves the overall efficiency as well as reducing maintenance costs or downtime. In addition, the systems can be made less expensive, more reliable, and lighter.

In an example embodiment, the electrically-actuated linear pumps use a planetary gear train (e.g., planetary gears surrounding a threaded rod to convert rotational motion of the planetary gears to the linear translation movement of the threaded rod) to linearly move (i.e., translate) plunger rods. The planetary gear train is powered by at least one electric motor, instead of the traditional diesel engines. The electricity supplied to the planetary gear train may be provided from the grid or produced by an onsite generator using local natural gas, thus minimizing operation fueling costs.

In addition, the linearly moving plunger rod may use both ends and couple with two fluid ends. Such configuration provides a redundant pumping axis as well as making the overall system lighter, which makes transport easier. This arrangement also eliminates the use of a diesel engine, the transmission attached thereon, and the cooling system associated therewith, as well as the pump power end. Besides improving the mechanical efficiency due to the reduction of moving parts, the electric linear pumps may be powered by natural gas electric generators, such as gas turbine generators, thus minimizing energy losses during conversion. The natural gas electric generators also produce less emission pollutants and less noise than diesel engines.

The linearly moving plunger rod has a much longer stroke length than the traditional plungers powered by diesel engines. In this way, maintenance costs are reduced because the number of valve reversals for a given pump flow rate is reduced. Similarly, the long stroke length also reduces the number of pressure cycles on the fluid ends for a given flow rate. Less cycles results in greater reliability and fewer failures of components.

This disclosure presents, in an embodiment, an electrically-actuated double acting pump that provides pumping action along the axis of its rod. The electric actuator may be one or more motors that provide rotational motion to a planetary gear train designed to drive a threaded rod linearly along its axis. The threaded rod includes plunger sections on both ends such that when the rod moves in either direction, one of the two ends will pump out fluids while the other draws in fluids. In other embodiments, the electric actuator may be in the form of a winding that uses electric current to create a magnetic field to move the rod along its axis (e.g., similar to solenoid actuation). A fluid end is coupled with each of the two plunger ends to control fluid charging on the suction stroke and pressure discharge on the power stroke.

In some embodiments, electrical actuation of the pumping element (rod, plunger, etc.) allows for a large variation of stroke lengths, such as from 1 meters to 3.5 meters, for example 2 meters. Longer stroke length provides an advantage by reducing the number of fluid end valve reversals and fluid cylinder pressure cycles on the fluid cylinder for a given flow rate or a given quantity to be pumped. In some embodiments, the electric drive can provide sufficient rod load to create 15,000 psi with existing frac plunger sizes.

The disclosed system and control method includes a control module, that is enabled using a computer with associated software installed therein, to provide smooth flow output as well as minimized fluid pulsation. As such, the operation of such electric linear pumps are much quieter than traditional operations using diesel engines and reciprocating pumps. In some embodiments, the pumping power may be in excess of 4000 hydraulic horsepower. In some instances, the plunger diameter may be up to 7.5″. The rod loads may be greater than 200,000 pounds-force.

In some embodiments, multiple pump units, such as from three to six units, may be used for redundancy and may be configured to maintain a constant or steady output flow (i.e., smooth output). In different implementations, different plunger sizes and fluid end sizes (e.g., different product families) may be provided for a range of pressures needed for different applications. In some cases, the electric linear pump have a stroke length of approximately 2 meters.

In some examples, the motor used in the linear electric pump may be a permanent magnet synchronous motor. The bearing may be a spherical axial thrust bearing. The planetary gears may directly drive a threaded plunger rod without additional transmission assemblies. In some cases, the electrically-actuated linear pump may have one or more sensors to measure the rotary position of the motors, and/or components of a drive mechanism, such as a gear ring, and/or the planetary gear train to determine position, speed, or other information of the plunger rod. This may be an improvement over hydraulically-actuated linear pumps where rotational position cannot be measured and converted to a known linear position of the plunger rod.

The motors, gear ring, planetary gears, bearings, and the threaded rods may be enclosed within a housing for lubrication and cooling purposes. Cooling system is provided for both the electric motor and the driven gears thereof. A logic control unit (LCU) may be used to accurately control the rotation of the motors and provide control according to control signals per control algorithm or programs. Detailed examples are provided below.

FIG. 1 is a top view of an electrically-actuated linear pump system 100 employing three electrically-actuated linear pumps 101, 102, and 103 arranged in an example configuration to be carried on a trailer. For example, the pump system 100 may be laid flat on the trailer bed, which may be 48 feet in length and up to 8.5 feet in width (oversized configurations are possible). The electrically-actuated linear pumps 101, 102, and 103 may be controlled to work in parallel by a common controller to produce a smooth output flow of fluids. The smooth output flow of fluids may be produced by shifting or adjusting individual speeds, phases, and/or flow rates of each of the electrically-actuated linear pumps 101, 102, and 103.

Each of the electrically-actuated linear pumps 101, 102, 103 includes a linear actuator housing 10 disposed at the center of the pump 101. The components on each side of the linear actuator housing 10 may be the same such that the pump 101 is a double-acting pump, and is either in a suction phase or a discharge phase, depending on the direction of displacement of the respective plungers. A left side of the electrically-actuated linear pump 101 includes a plunger housing 12 a coupled to the linear actuator housing 10, and a fluid end 14 a coupled to the plunger housing 12 a. Similarly, a right side of the electrically-actuated linear pump 101, includes a plunger housing 12 b, coupled to the linear actuator housing 10, and a fluid end 14 b coupled to the plunger housing 12 b. The pumps 101, 102, and 103 are shown in a staggered configuration, with the middle pump 102 position slightly forward of the pumps 101 and 103, but the pumps could also be configured as one pump 102 stacked on top of the other two pumps 101 and 103.

Each of the electrically-actuated linear pumps 101, 102, and 103 include the same components and operate the same as each other, so this disclosure will only describe pump 101, and such description applies to pumps 101, 102, and 103. FIGS. 2A, 2B, and 2C are schematic diagrams of the right side of the electrically actuated linear pump 101 shown in cross-section to illustrate the inner workings of the pump 101. The left side is identical, and thus the right side description applies to the left side. FIG. 2A shows a double-acting rod 16 in a centered, neutral configuration. The double-acting rod 16 is linearly displaced along its axis by a linear actuator, for example a motor driven planetary gear train, as described in further detail below. FIG. 2B shows the double-acting rod 16 displaced towards the left fluid end 14 a and away from the right fluid end 14 b. Thus, FIG. 2B shows pumping fluid from the left fluid end 14 a in a power stroke and drawing fluid into the right fluid end 14 b in a suction stroke. FIG. 2C shows the pump 101 with the double-acting rod 16 displaced toward the right fluid end 14 b, and away from the left fluid end 14 a, such that the fluid is pumped from the right fluid end 14 b and simultaneously drawn in the left fluid end 14 a. According to an embodiment, the plunger rod 16 has a total length of 7.5 meters operating at a 2 meters stroke. The electric motor(s) 24 uses a planetary gear train (also referred to as a planetary screw drive) to move the threaded center portion.

According to certain embodiments, the displacement of the double-acting rod 16 is considerably greater than the displacement of a plunger in a reciprocating pump, thus the pump 101 cycles fewer times to produce a similar flow rate. Fewer cycles on the pump reduces wear on certain components. According to certain embodiments, the double-acting rod 16 may have a displacement stroke of approximately two meters either to the left or to the right. Different length strokes are contemplated by this disclosure.

For example, increasing plunger sizes requires more power to pump at a given pressure. Different plunger sizes may be employed to increase flow rate (larger diameter plungers) or to increase pumping pressure (smaller diameter plungers). The longer the stroke of a linear pump, the more power it is able to convert to pumping power (or fluid power) because each time the pumping axis reverses direction, the axis needs to ramp down in speed, stop, and then ramp back up in speed. During this time of reversal, the power is limited to an average of about 50% of the full rating. However, when the axis reaches its maximum constant speed the power output is 100% of the full rating. The maximum stroke of an axis can be limited by its buckling strength, or at least limited to the maximum length of the trailer. The shorter the stroke of an axis, the faster the pump needs to cycle to create the same rate as a longer stroke. Higher numbers of reversals are less efficient and also create more pressure cycles on valves, seats, packing, fluid ends, etc. Also, reversing direction too quickly (high cycle rates) can create high peak motor current which shortens the life of the electric motor. The life of the roller screw assembly, such as the drive mechanism described in more detail below, is a balance between load and speed.

The double acting rod 16 includes a screw portion 18 (also referred to as a threaded portion, a screw, or a screw portion) coupled on respective ends to a left plunger portion and a right plunger portion 20. According to certain embodiments, the plunger portions 20 may be integral with the screw portion 18. In other embodiments, the plunger portions 20 may be mechanically or otherwise joined to the screw portion 18. The screw housing 22 protects the screw portion 18 from damage and other environmental hazards that may interfere with the operation of the screw portion 18. According to certain embodiments, the screw housing 22 may be integral with the linear actuator housing 10. According to an alternate embodiment, the screw housing 22 may be formed separately and mechanically secured to the linear actuator housing 10.

The respective plunger housings 12 a, 12 b are coupled to each end of the respective ends of the screw housing 22. Seals are positioned between the double acting rod 16 and the plunger housing 12 a, 12 b and/or the screw housing 22 to seal the screw housing 22 and the screw portion 18 from exposure to the fluid being received and discharged, for example hydraulic fracturing fluid, by the pump 101.

The linear actuator housing 10 houses and protects the components that drive the double acting rod 16, such as at least one electric motor 24 and components associated with a screw drive mechanism 26, such as a planetary gear train, as described in more detail with respect to FIG. 3. This disclosure contemplates any suitable electrically-powered drive mechanism that is capable of linearly displacing the double-acting rod 16, for example one or more electric motors.

An overall length of the pump 101 including the fluid ends 14 a, 14 b may be approximately 11.5 meters. For example, the screw housing 22 may extend approximately 2 meters (or one stroke length) in each direction from the linear actuator housing 10, which may be approximately one meter in length along the axis of the rod 16. Each plunger housing 12 a, 12 b extends approximately two meters (one stroke length) from the screw housing 22. Each fluid end extends about one meter from the plunger housings 12 a, 12 b. A shorter or longer stroke length may shorten or lengthen the overall length of the pump 101.

FIG. 3 is a schematic diagram of the components within the linear actuator housing 10 with portions broken away to show the internal workings of the screw drive mechanism 26 and its interaction with the screw 18. One or more electric motors 24 are supported and spaced circumferentially apart around the double acting rod 16. According to one embodiment, seven electric motors are circumferentially spaced apart about the rod 16, and more specifically about the screw portion 18 of the rod 16. Using multiple smaller motors 24 may be less expensive than using fewer, for example one, larger motor(s). The number of motors and their size may vary to fit the desired power and/or load.

The motors 24 are electrically powered and electronically controlled to drive the rotation of a nut 28 with at least one gear ring 30. The motors 24 share the torque load. A support bearing 34 supports a plurality of rollers 32 and maintains their circumferential spacing around the screw 18. FIG. 3 shows a subset of the rollers 32. According to an embodiment, a planetary gear train includes a plurality of rollers disposed circumferentially about the screw 18, which functions as a sun gear. Each roller has a threaded portion 35 that is in meshed engagement with the threads of the screw 18. The nut 28 is rotationally driven by the motors 24, which drives the rollers 32. The rollers 32 revolve about the axis of the screw 18 and rotate about their individual axes.

The gear ring 30 includes gear teeth that mesh with corresponding gear teeth 37 of each roller 32, which may be disposed at each end of an elongated roller 32. The planetary motion of the rollers around the screw 14 induced by the nut 28 and the gear ring 30 directs the linear motion of the rod 16 in a worm-drive configuration. The screw 18 is constrained from rotation, so the rollers 32 linearly displace the screw 18. The direction of displacement is determined by the direction of rotation of the motors 24 and the direction of rotation of the nut 28, gear ring 30, and the rollers 32. Reversing the direction of rotation of the motors 24 reverses the rotational direction of the nut 28, gear ring 30, and the rollers 32 such that the screw 18 will linearly translate in the opposite direction.

The pump 101 may be secured to a frame (not shown) by attaching a foot portion of the linear actuator housing 10 to the frame. The frame attachments constrain the linear actuator housing 10 from rotating and from linear motion in the direction of the axis of the rod 16. Thus, when the electric motors turn the nut 28, the gear ring 30, and the rollers 32, the screw is displaced either left or right depending on the rotational direction of the motors 24. An inverter for each motor 24 may convert the AC power into DC power to power a DC permanent magnet embodiment of the motor 24.

FIG. 4 is a schematic diagram of a power system 50 employed to supply power to one or more of the pumps 101, 102, 103, of the electrically-actuated linear pump system 100. A turbine generator 52 is often available at a frac site and is provided by a conventional E-frac fleet. According to one embodiment, the turbine generator 52 may run on gas, such as natural gas, to rotate one or more turbines to generate AC electrical power at a high voltage, for example 13,800 Volts. The voltage from the generator 52 is stepped down by a transformer 54 to a voltage that can be used by an inverter. The stepped down voltage is converted to DC power by the inverter such that the DC power can be delivered to each motor 24 of each pump 101, 102, 103. A linear actuator 25 is electrically coupled to the generator 52 and in communication with the control unit 70. According to one embodiment, the linear actuator may include at least one DC permanent magnet motor, for example seven motors, that mechanically power the drive mechanism 26, as described above with respect to FIG. 3. Together the motors and the drive mechanism, including the threaded rod may be referred to generally as a linear actuator. According to one embodiment, each motor may be coupled to a dedicated inverter. The turbine generator 52 and the transformer 54 may be located elsewhere on the frac site and not on a particular trailer.

The pump 101 is also fluidly coupled to a lubrication system 60 that includes lubrication conduits 62 and a lubrication cooler 64. The lubrication system 60 supplies lubrication oil to cool and lubricate the mechanical components of the pump 101. The lubrication cooler 64 removes heat and cools lubrication fluid that has been heated by the friction between mechanical components.

The control scheme allows for producing steady or constant output flows in an array of electrically-actuated linear pumps 101, 102, 103 discussed above, to remove flow fluctuations when the plunger rod is reversed in direction. According to an example embodiment, a first plunger rod is powered to move at a first speed. Simultaneously, a second plunger rod of another electrically-actuated linear pump is actuated at a second speed. A control unit 70 operates the first and the second plunger rods to maintain an overall output flow rate. The control unit 70 may include an inverter, power supply, and/or a logic controller programmed to execute an algorithm to control the phases of the duty cycles of multiple electrically-actuated linear pumps to maintain a constant flow rate.

According to an embodiment, the control unit 70 adjusts the phases and/or magnitudes of the duty cycles of the first and the second plunger rods to remove unwanted effects when one of the first or the second plunger rod is reversed in direction, such as at the end of each of the reciprocating strokes. The control unit 70 may control the duty cycles of three or more than three pumps delivering fluid to a wellbore. As such, a constant flow rate may be maintained. An example of a logic scheme to maintain a constant flow rate that can be employed according to the teachings of the present disclosure is shown and described in International Patent Application Publication No. WO 2020/037283 assigned to S.P.M. Flow Control, Inc. and entitled “Actuator for a Reciprocating Pump,” which is hereby incorporated by reference.

Although only two plunger rods are discussed, in practice or other embodiments, the controller may control three or more plunger rods in similar fashions. In some cases, three, six, or more plunger rods are controlled. In some embodiments when three or more plunger rods are in operation, the controller can adjust the speeds of plunger rods even when one of the multiple plunger rods completely fails, such as by increasing the speeds of the remaining plunger rods and adjusting the phases accordingly.

FIG. 5 is a perspective view of an electrically-actuated linear pump 80 according to the teachings of the present disclosure. The pump 80 may be one of an array of pumps 80 supported by a trailer. The pump 80 includes a plurality of motors that receive electrical power, for example DC power, from a power source, such as a turbine generator. The motors supply rotational motion to a drive system housed in a drive housing 84 that linearly displaces a plunger to the left and to the right depending on the direction of motor rotation. The plunger displacement either draws in fluid in a suction stroke, for example hydraulic fracturing fluid, or discharges the fluid in a power stroke, depending on whether the plunger is retreating away from or advancing toward a respective fluid end 86 a, 86 b.

The pump 80 includes the plurality of electric motors, for example up to seven electric motors, operatively coupled to a drive mechanism housed within a drive mechanism housing 84. According to one embodiment, the drive mechanism includes the components of the planetary gear train described above with respect to FIG. 3. A screw housing 88 houses and protects a threaded screw portion of the rod and is coupled to the drive mechanism housing 84 and together the screw housing 88 and the drive mechanism housing 84 house the screw portion of the rod. A first plunger housing 90 a is coupled to one side of the screw housing 88, and a second plunger housing 90 b is coupled to the other side of the screw housing 88. As described in more detail above with respect to FIGS. 2A-2C, the plunger housings 90 a, 90 b house the plunger portions of the linearly displaceable double-acting rod and act as pumping chambers. A first fluid end 86 a is coupled to the first plunger housing 90 a, and a second fluid end 86 b is coupled to the second plunger housing 90 b. Each fluid end 86 a, 86 b includes a suction inlet 92 a, 92 b and a discharge 94 a, 94 b. According to one embodiment, an inlet check valve is disposed proximate each suction inlet 92 a, 92 b and a discharge check valve is disposed proximate each discharge 94 a, 94 b. The discharge is fluidly coupled to a discharge line 96 a, 96 b. The discharge line is fluidly coupled to a wellbore. According to an embodiment, the discharge lines of multiple pumps 80 are fluidly coupled by a wye 99 such that the fluid can be delivered to the wellbore at a constant rate.

One or more of the housings may be coupled to one or more mounting skids 110 a, 110 b. For example, one end of the screw housing 88 may be coupled to a first mounting skid 110 a, and the other end of the screw housing 88 may be coupled to a second mounting skid 110 b. The mounting skids support the pump on the trailer and maintain the position of the housings to allow the linear motion of the double acting rod to draw in and discharge the fluid in the fluid ends without moving the pump.

In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose.

In the specification and claims, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear.

In addition, the foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and/or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.

Furthermore, invention(s) have described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention(s), as defined solely by the appended claims. Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment. 

What is claimed is:
 1. An electrically-actuated linear pump, comprising: a linear actuator housing; a screw housing coupled to the linear actuator housing; a rod configured to linearly translate within the screw housing, the rod comprising a threaded portion disposed axially between a first plunger portion and a second plunger portion; a linear actuator disposed within the linear actuator housing and comprising a drive mechanism in meshed engagement with the threaded portion of the rod, the meshed engagement operable to linearly translate the rod, the linear actuator being electrically-actuated; a first fluid end and a second fluid end disposed opposite the first fluid end, each fluid end configured to receive and discharge a fluid; and wherein translation of the rod toward the first fluid end discharges the fluid from the first fluid end and simultaneously draws the fluid into the second fluid end.
 2. The electrically-actuated linear pump of claim 1 wherein the linear actuator further comprises at least one electric motor operably coupled to the drive mechanism.
 3. The electrically-actuated linear pump of claim 2 wherein the at least one electric motor is a permanent magnet motor.
 4. The electrically-actuated linear pump of claim 1 wherein the linear actuator comprises a plurality of electric motors circumferentially spaced apart about an axis of the rod, each of the plurality of electric motors operably coupled to the drive mechanism.
 5. The electrically-actuated linear pump of claim 4 wherein the drive mechanism comprises a plurality of rollers spaced apart circumferentially about the axis of the rod, each roller in meshed engagement the threaded portion of the rod.
 6. The electrically-actuated linear pump of claim 1 wherein the drive mechanism comprises a planetary gear train.
 7. The electrically-actuated linear pump of claim 6 wherein the linear actuator further comprises a plurality of electric motors, each operable to drive the planetary gear train.
 8. The electrically-actuated linear pump of claim 1 further comprising a first plunger housing coupled to one end of the screw housing and a second plunger housing coupled to an opposite end of the screw housing, the first fluid end being coupled to the first plunger housing and the second fluid end being coupled to the second plunger housing.
 9. The electrically-actuated linear pump of claim 1 wherein the rod functions as a sun gear in a planetary gear train.
 10. The electrically-actuated linear pump of claim 1 further comprising an inverter operable to convert AC power to DC power, the DC power energizing the linear actuator.
 11. A pump system, comprising: a first electrically-actuated linear pump and a second electrically-actuated linear pump, each comprising: a linear actuator housing; a screw housing coupled to the linear actuator housing; a rod configured to linearly translate within the screw housing, the rod comprising a threaded portion disposed axially between a first plunger portion and a second plunger portion; a linear actuator disposed within the linear actuator housing and comprising a drive mechanism in meshed engagement with the threaded portion of the rod, the meshed engagement operable to linearly translate the rod, the linear actuator being electrically-actuated; a first fluid end and a second fluid end disposed opposite the first fluid end, each fluid end configured to receive and discharge a fluid; and wherein translation of the rod toward the first fluid end discharges the fluid from the first fluid end and simultaneously draws the fluid into the second fluid end and translation of the rod toward the second fluid end discharges the fluid from the second fluid end and simultaneously draws the fluid into the first fluid end in a duty cycle; and a controller configured to control outputs of the first and second electrically-actuated linear pumps.
 12. The pump system of claim 11 wherein the duty cycle of the first electrically-actuated linear pump is out of phase with the duty cycle of the second electrically-actuated linear pump.
 13. The pump system of claim 11 wherein the linear actuator of each of the first and second electrically-actuated linear pumps further comprises a plurality of electric motors circumferentially spaced apart about an axis of the rod, each of the plurality of electric motors operably coupled to the drive mechanism.
 14. The pump system of claim 13 wherein the controller is configured to control the outputs of at least three electrically-actuated linear pumps.
 15. The pump system of claim 11 wherein each of the first and second electrically-actuated linear pumps further comprises a first plunger housing coupled to one end of the screw housing and a second plunger housing coupled to an opposite end of the screw housing, the first fluid end being coupled to the first plunger housing and the second fluid end being coupled to the second plunger housing.
 16. A method of delivering a fluid to a wellbore, comprising: rotating at least one electric motor in a first rotational direction, the at least one electric motor operably coupled to a drive mechanism of a linear pump, the linear pump, comprising: a linear actuator housing; a screw housing coupled to the linear actuator housing; a rod configured to linearly translate within the screw housing, the rod comprising a threaded portion disposed axially between a first plunger portion and a second plunger portion; a linear actuator disposed within the linear actuator housing and comprising the at least one motor and the drive mechanism, the drive mechanism in meshed engagement with the threaded portion of the rod; and a first fluid end and a second fluid end disposed opposite the first fluid end, each fluid end configured to receive and discharge a fluid; wherein rotation of the at least one electric motor in the first direction translates the rod toward the first fluid end to discharge the fluid from the first fluid end and simultaneously draw the fluid into the second fluid end; and rotating the at least one motor in a second rotational direction opposite the first rotational direction, wherein rotation of the at least one electric motor in the second direction translates the rod toward the second fluid end to discharge the fluid from the second fluid end and simultaneously draw the fluid into the first fluid end.
 17. The method of claim 16 further comprising sensing a rotational position of the drive mechanism.
 18. The method of claim 16 wherein rotating the at least one motor in the first rotational direction displaces the rod about two meters.
 19. The method of claim 16 wherein the at least one electric motor comprises a plurality of electric motors circumferentially spaced apart about an axis of the rod, each of the plurality of electric motors operably coupled to the drive mechanism.
 20. The method of claim 19 wherein the drive mechanism of each of the first and second electrically-actuated linear pumps comprises a planetary gear train. 